1. Field of the Invention
The present invention relates to a technique for measuring the amount of toner adhesion in a toner image formed on a bearing member of an image forming apparatus.
2. Description of the Related Art
Colors of images formed by an electrophotographic image forming apparatus vary with changes in various physical parameters, even if apparatus settings for image forming are unchanged. In particular, development and transfer processes typically contribute to variations in colors. This is because the amount of toner adhering to a photoconductive drum or a transfer belt is not stable, since environmental changes, such as changes in temperature and humidity, cause changes in latent image potential, the amount of toner supply, and transfer efficiency. Therefore, it is necessary to measure the amount of toner adhering to the photoconductive drum or transfer belt, control the amount of light exposure, development voltage, and transfer current in accordance with the measurement result, and thereby stabilize the development and transfer processes.
In general, such a control operation is performed when the printer environment changes, such as after replacing a toner cartridge, after printing a predetermined number of sheets, or after turning on the power of the printer main body. For measurement of the amount of toner adhesion, a plurality of toner patches with various densities (from low to high) are formed on the photoconductive drum or transfer belt. Then, after a toner adhesion measuring device measures the amount of toner adhesion in these toner patches, various control operations are performed under appropriate image forming conditions, in accordance with the measurement result.
Japanese Patent Laid-Open No. 8-327331 and Japanese Patent Laid-Open No. 4-156479 describe methods for measuring the amount of toner adhesion. Japanese Patent Laid-Open No. 8-327331 discloses a method in which the amount of light reflected from a bearing member irradiated with light and the amount of light reflected from a toner patch irradiated with light are detected, the amount of toner adhesion is measured from a difference between the detected amounts of reflected light, and image density parameters are controlled in accordance with the measurement result. Japanese Patent Laid-Open No. 4-156479 discloses a method in which the amount of toner adhesion is detected by measuring the thickness of a toner patch with a toner displacement gauge. In this method, an image bearing member and a toner image are irradiated with spot light, an image of reflected light is formed at a position corresponding to the thickness of the toner patch adhering to the image bearing member, the amount of toner adhesion is measured from a change in the position of the formed image, and image density parameters of the image pickup system are feedback-controlled on the basis of the measured thickness.
When the amount of toner adhesion is measured from the light reflection position, it is important to ensure that the surface of an object to be measured is optically uniform. That is, even if the same laser is used to irradiate the bearing member with spot light having the same power and diameter, micro-irregularities or scratches on the surface of the bearing member can cause uneven reflection within the spot, and can create distortion in the reflection waveform detected by an image pickup element. This makes it difficult to accurately detect the position of the reflection waveform and increases errors in measured values.
An actual waveform of laser light reflected from a bearing member is illustrated in FIG. 11. FIG. 11 illustrates a reflection waveform picked up by a line sensor. The irradiation of laser light originally has a bell-shaped distribution (Gaussian distribution) which is highest (brightest) at its center. However, in the example of FIG. 11, scratches on a belt surface cause unevenness in the amount of reflected light. As illustrated, the waveform has a bright line which rises sharply (due to a scratch) at a position to the right of the center. When performing detection of a center position of the reflected light using data of the entire spot, the line sensor determines, due to a significant effect of the bright line, that the center position is located to the right of the original center position of the laser spot. Therefore, a high toner height value is output as a detected value. Basically, a relative position of a scratch within a spot changes randomly depending on the installation of the sensor, fluctuations in the belt main scanning direction, and various types of mechanical vibration. Therefore, distortion of the picked-up waveform and the detected intensity values associated therewith are output as random noises that randomly change. In particular, when the scratch passes through the center of the spot, the scratch is intensely irradiated with the brightest light at the center. This tends to cause an increase in waveform distortion and error.